Presently we can relate over half of respiratory tract illnesses in infants and children to some one of the myxoviruses, adenoviruses or Mycoplasma pneumoniae. Whether the relationships with illness and ecology of these agents will change through the years is unknown. The role of other agents recently uncovered or not yet uncovered remains to be determined. Little is known on the pathogenesis of infection with these agents, particularly with respect to factors of host resistance and the role of local versus serum antibody in protection or in production of disease. In order more fully to understand these infections and their possible prevention we propose: (1) to determine (a) the relationship of non-bacterial agents to respiratory tract illness and (b) the clinical relationship of these agents. (2) To determine the pathogenesis of infection and development of resistance to respiratory tract viral infection. (3) To study the natural history of viral and mycoplasma agents.